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Flashcards in Intracellular trafficking Deck (15):

What are the proteins that receive post translational signals and describe the basis.

Cytoplasmic proteins (cytoskeleton) have no signal.


Nucleus - nuclear proteins enter the bidirectional pores. But nuclear proteins are transported because of its signal sequence. They sequence is recognized by cytoplasmic importin proteins. This allows it to be transported through the pore. Then, Ran bound to GTP binds to the complex (GEF is in the nucleus) and the nuclear protein is released.

Peroxisomal proteins: also have a signal which is recognized by the peroxisome signal sequence receptor so it can enter the peroxisome



Describe Zellweger Syndrome.

The signal sequence is mutated so it can't be recognized by the receptor and enter the peroxisome.


Describe the secretory protein synthesis pathway.

The proteins that enter this pathway are signaled cotranslationally.

Cytoplasmic ribosomes come together with secretory protein mRNA. SRP (signal recognition particle) binds to the N-terminus of the protein. This halts protein synthesis,

There is an SRP receptor on the membrane which binds the ribosome/protein and translation continues.

GTP hydrolysis dissociates SRP and SRP receptor.

The receptor is almost like a channel so when the protein enters the cell, the signal peptidase cleaves the signal peptide (so it is unidirectional) and then the protein starts to form and sugar chains are placed at specific sites to begin the processing). Essentially the sugar is only placed on if the protein is properly folding, otherwise it can't leave the ER.

The oligosaccharide is then modified with each sequential compartment.


Describe vesicular transport and its requirements.

Vesicular transport is transport of cargo without crossing the membrane.


1. form vesicle - protein coats are needed to deform the membrane
a. COP1 and COPII (coatamer) are regulated by G proteins.
ARF for COP-1 (retrograde) and Sar-1 for COPII.

2. there must be a way to specifically select for the cargo

3. they must have a way to distinguish their target (via SNAREs)


Describe how Sar-1 mediated vesicle formation for anterograde transport occurs.

Sar1-GDP has a hidden fatty acid tail. When GDP is exchanged to GTP, the lipid is exposed and can insert into the ER membrane. Then different subunits bind which can themselves bind to cargo receptors and the cargo molecules.


Describe vesicular address localization in specific detail (fusion)

and toxins.

Each vesicle has a specific v-snare that will only bind to a specific t-SNARE on the target compartment.

There are also specific G proteins called Rabs that also help recognize targets. If the SNAREs are a correct fit, a coil coil form and the vesicle fuses.

Botulinum toxin and tetanus toxin cleave snares and prevent neurotransmitter release.


Summarize the formation and fusion of vesicles.

G proteins will bind to specific receptors on the membranes. This will bind COAT subunits.

Cargo is recognized by cargo receptor proteins.
The receptors are recognized by the Coat proteins.

The initiation of the budding phase requires GTP binding proteins

After release, the coat dissociates and the snares are exposed.

Rab and snare mediate fusion to the target.


Describe the retrieval process.

If proteins escape the ER if they are to remain in the ER, there are signal sequences (KDEL) which will be returned to the ER.


Describe the golgi apparatus

There is a mixture of proteins in the ER and golgi network. There is only sorting when the proteins reach the trans golgi network. There, the modifications are recognized by receptors on specific regions of the TGN membrane. For example, mannose-6 phosphate on lysosomal proteins is recognized by mannose-6 phosphate receptor.


Describe lysosomal protein transport.

Lysosomal proteins are marked in the cis golgi by phosphotransferase which will add a phosphate to the mannose sugar.

Mannose 6 phosphate is recognized by the receptor and then bud off, and fuse to the late endosome. The acidic environment will cause the receptor and protein to dissociate. Then this will go on to become the lysosome.


What occurs in human I-cell disease?

The mannose-6-phosphate modification doesn't occur because the phosphotransferase is mutated. Lysosomal proteins cannot be recognized into vesicles bound for lysosomes and instead they all enter the constitutive secretory pathway (default pathway with no sorting) .


Describe legionairre's disease

The bacteria hijacks the host vesicles and interferes the phagosome from going to the lysosome. This bacteria will live inside the phagosome and mimic the RER. Then it can continue to replicate.


Describe endocytosis and its three possible routes. One of them has a specific example.

Endocytosis is mediated by clathrin coated pit.

-clathrin does not bind directly to cargo receptors, instead it is mediated by an adaptin protein molecule. A mutation in the receptor tail would interfere with adaptin recognition and so not endocytosis would occur.

All of these vesicles are given to early endosome, the sorting station much like the trans golgi network.
Three pathways:
1. Receptors can be recycled back to to the apical membrane.
2. transcytosis - bypasses tight junctions. It is used to give antibodies to neonates from mother milk
3. Consititutive pathway: the receptor gets degraded in the lysosome and cargo is released.


Describe the LDL endocytosis pathway.

And disease state.

and EGF

LDL is contains cholesterol. ON the apical plasma membrane, LDL will bind to LDL receptor.

A coated pit of clathrin will form, the coat with dissociate before it fuses with the endosome. The acidic environment dissociates the LDL receptor and the recycling endosome carries the receptor back to the apical membrane.

The early endosome continues with the LDL to the lysosome where LDL is degraded and free cholesterol is given to the cell. This is important for transporting cholesterol through the blood.

ADDITIONALLY: EGF receptor is different, it goes down the constitutive endocytosis pathway and it gets degraded in the lysosome and is an example of receptor down regulation.

MUTATION: in the LDL receptor tail doesn't allow adaptin to bind and so clathrin can't bind. Hypercholesterolemia occurs (where there is high circulating cholesterol).


How do viruses use biosynthetic pathways.

Viruses have envelop proteins which are recognized by receptors on the plasma membrane. A coated pit forms and then a coated vesicle fuses with the endosome. This releases the capsid protein.

The capsid disassembles due to acidic endosome revealing the virus RNA which has to make envelop proteins and capsid proteins.

Also viral RNA is replicated.

Envelope proteins are made with an SRP signal sequence and enter the secretory pathway. They enter the plasma membrane. When capsid meets the envelope protein, There is budding and the virus goes on again.